[FFmpeg-user] ffmpeg & MPEG-2 GOP structure

[Valentin NOEL]

2015-01-09 18:00 GMT+01:00 tim nicholson <nichot20-at-yahoo.com at ffmpeg.org>:

> On 09/01/15 15:30, Valentin NOEL wrote:
> > 2015-01-09 15:29 GMT+01:00 tim nicholson <
> nichot20-at-yahoo.com at ffmpeg.org>:
> >
> >> [...]
> >> It means that I was making a pragmatic suggestion for something you
> >> could try immediately to solve your issue, rather than trying to get
> >> bogged down in technical details too early.
> >>
> >
> > Actually, I need to add this flag "+cgop" because I need to generate
> closed
> > GOP.
> > Other encoders allow finishing closed GOP with B-frames (with only
> backward
> > reference), so I wonder whether FFmpeg cannot allow doing this, or
> whether
> > I do not use it correctly.
> >
>
> Well does toggling that option off change things at all?
> (Notwithstanding that for real use you need it, just trying to see what
> might affect the result as I know chaging this can affect the bframe
> count).
>

Yes, it generates a first GOP of 10 frames : I B B P B B P B B P
The following GOPs are containing 12 frames : B B I B B P B B P B B P
Checking the gop headers in the bitstream, it appears that all the GOPs are
open, even the first (I would think it is closed).


> -b_strategy  0 is the default so that setting should be doing nothing,
> have you experimented with different values?
>

-b_strategy 1 alone gives : I P P P P P P P P P P P P
but it depends on the setting -b_sensitivity (I didn't find what the
default value is) :
    - values from 1 to ~4 : some B-frames appear sometimes (decreasing with
the option value) and variable GOP length
    - values from ~4 and more : I P P P P P P P P P P P
-b_strategy 2, does not give better results but much longer process : I B P
B B P B B P B B P

Results are quiet the same without the +cgop flag.

>
> >>> Thanks.
> >>>
> >>>[....]
>
>
> --
> Tim.
> Key Fingerprint 38CF DB09 3ED0 F607 8B67 6CED 0C0B FC44 8B0B FC83
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